1. Field of Invention
This invention allows multiple users to access a single signature device located on a central network computer by typing a unique passphrase known only to the individual user and presenting a digital certificate that contains personal data about the individual user and to sign using the certificate information which is also incorporated into the signature itself as a way to authenticate the user in connection with the signed electronic document or data.
2. Description of Prior Art
The dominant technology under prior art for individuals to sign electronic documents and transaction data is based upon client-side digital signatures. The signatures are created by software that uses an encryption algorithm, called a private key, of the user to electronically encode the electronic document or transaction data. A mathematically related algorithm, called the public key corresponds to the private signature key. The public key is used by the recipient to verify the authenticity of the electronic document or data and the integrity of the data since signing occurred, including the fact that it has not been changed or altered since the signature was affixed. Digital certificates issued by trusted third parties called certification authorities identify public keys of the presumptive true owners of the private keys that were used for signing, thus assuring that the signer is in fact the person who purported to sign the document or data.
An example of a system of digital signatures is shown in U.S. Pat. No. 4,405,829 to Rivest et al. (1983). It is based upon a technology commonly referred to as xe2x80x9casymmetric encryption.xe2x80x9d In this technology, a user generates two mathematically related numbers based upon prime numbers, called keys. The so-called private key remains with the issuing user. It is kept secret. The other key, denominated the public key, can be freely distributed by the issuer to others. The keys are related, but they are not identical. They perform reverse roles. One is used to encrypt information, and the other to decrypt it. With respect to signatures, one key affixes the signature and other is used to verify it and the electronic document contents.
Electronic communications are signed, generally with the private key, in a two step process. First a digest of a message is created with a one way hash function, and then the hash function is encrypted using the private key. The authenticity of the message and its contents can be verified by a recipient as being authentic and sent from the signing party by testing the message using the public key. An altered message or fraudulent sender will be detected by a computer possessing the proper software and the public key. If either the message has been altered since signing or alternatively the signer did not use the proper private key, the signature will be reported as false or inauthentic. This method is useful for electronic authentication.
However, to the extent that this method of authentication occurs using individual desktop or laptop computers that are identified to others through a system of digital certificates, it also requires a massive infrastructure for key management and verification by trusted third parties, called certification authorities. These certification authorities verify the identities of individual key holders before issuing certificates to them. Once identity is confirmed, they sign the public key of the individual with the certification authority""s private key. They also allow others to verify that the public key of the key pair belongs to the party who is identified as the holder of the key pair, and maintain lists of active and revoked certificates for use by third parties that rely upon the certificates to prove identity. Authentication by a relying party requires not only a check of the digital signature on the message, but also of the status of the certificate identifying the signer, to make sure that it is still valid. This involves accessing the certificate authority computer and checking its lists of revoked and suspended certificates. The investment to create and operate a commercial or large enterprise-wide certification authority is considerable. Legal requirements of periodic audits impose other costs.
The digital certificates from certification authorities identify the owner of the key pair principally through the owner""s public key that was signed by the certification authority at the time of issue. No other identification is made part of the certificatexe2x80x94no picture identification, fingerprint identification, handwriting exemplar, voice print, finger print, retinal scan, or other additional proof in the certificate of the owner""s personal identity. Without such other proof as part of the certificate itself, there is no personal identification of the owner to protect the certificate from subsequent wrongful use. An identity check is performed by the certification authority at the time that the public key is signed, but not afterwards. This makes it possible for an unauthorized person who comes into possession of the private key and the certificate of another to claim the identity of the true owner for purposes of one or more transactions over the Internet. The assumed identity can continue until the wrongful use is discovered and the certificate is revoked by the certification authority. Under the laws of many states, the true owner could be bound to a transaction involving wrongful certificate use up to the moment of certificate revocation because there is no other proof of identity needed or required to complete a transaction other than possession of the private key that corresponds to the public key which was signed by the certification authority. This risk is usually placed upon the key owner by contractual agreement, governing law, or custom, and may be protected against by insurance or warranty coverage.
Obtaining possession of the private key without authorization of the owner is not impossible using currently available technology. Private keys left on the hard drive of the owner""s computer are subject to various computer attacks. Because the true owner gains access to the private key on the computer""s hard drive generally using an unencrypted password, anyone who can learn or decipher this password has equal access. A password can be deciphered through a brute force dictionary attack. All possible permutations and combinations are generated electronically on another computer until the proper password is reconstructed. Generally, there is no check on the number of failed attempts to access the password, the public key or logging device built into the software.
One additional precaution that can be taken in light of this weakness is to store the private keys not on the computer hard drive, but on a card, called a smart card, which is read via a computer peripheral device. The card is inserted into the peripheral device only at the time of signature and is removed immediately thereafter. By keeping the key on the card and in the possession of all times of the owner, the security deficiency caused by leaving the key on the hard drive is corrected, but that is not the end of the matter. The smart cards themselves are subject to being stolen from the person of the owner, and there have been a number of successful attacks on smart cards in which the private key has been extracted in laboratory tests that simulate attacks by sophisticated computer hackers.
A certification authority infrastructure requires a massive investment in equipment and personnel that results in a relatively high cost to the user. The certification authority keeps track of the current status of the public keys that have been signed by it. Each public key corresponds to a private key that may be employed on the client computer for signature purposes.
The system is also particularly awkward for business enterprises. In the event of the compromise of a private key of an employee, all correspondents, including customers, must be alerted and replace that employee""s public key with a new one, which may be viewed as an annoyance or bother inappropriate for a business context. When employees leave, the employee certificates that were issued by a certification authority on behalf of the enterprise must also be revoked so that the authority contained in the certificate is no longer valid.
On a broader scale, because the integrity of the entire system of this prior art rests upon the security of the certification authority, if the private key of a commercial certification authority is ever compromised, all certificates issued by it are suspect, and a massive undertaking to replace all keys and certificates of users or certification authorities lower in the certification chain of trust is required.
Furthermore, unless suitable means are adopted by the certification authority to verify the true identity of a holder of a private key in the first place, the claimed security of all subsequent transactions involving the issued certificate is questionable. If less than thorough identity checks are initially made, the true identity of the key holder may be in doubt. As of the date of this application, there is no agreed upon legal procedure or standard for certification authorities, their agents, or registration authorities affiliated with the certification authorities to follow in making initial identity checks for purposes of issuing digital certificates to computer users.
Certificates intentionally have a limited life-span. At the end of the cycle, if the private key has not already been compromised, a new certificate is required. This is done for security reasons. As a result of this practice, real-time checking of the validity of the corresponding digital certificate from the certification authority is necessary, to make sure the certificate is still valid at the time of signing. Thus, the prior art creates a need for continuous communications between users and their certification authorities. This need can create enormous network traffic and bandwidth usage as significant numbers of transactions take place.
There are also significant liability issues which can arise from the need for prompt notice to potential relying third parties from the certification authority of compromised or revoked certificates. If such notice is not correctly given, a third party who relies upon a compromised private key may have a legal cause of action against the responsible certification authority for damages, costs and attorney""s fees incurred. This liability cannot be limited by contract in certain jurisdictions.
Furthermore, in business and legal settings where both parties are required to electronically sign documents, filings or transaction records using their respective private keys and digital certificates, and they are located in or claim citizenship of different legal jurisdictions or countries there is a possibility for uncertainty or actual conflict in the various laws that theoretically are applicable to the transaction. In some countries, users may be required to give copies of the private keys to the applicable governmental authority upon pain of punishment. This requirement may compromise the privacy and security of the electronic signatures. Where different legal regimes are involved, such uncertainty or conflict may actually impede the use of the electronic signatures for fear of participants of legal attacks by overzealous authorities or corrupt ones, depending on the reputations of the countries involved and their respective political regimes.
A widespread proliferation of private keys among individual computer users for electronic commerce is a basic tenet of the prior art. This creation and dispersion of private keys creates an administrative and legal tangle for those charged with keeping track of the corresponding public keys and the certificates which prove ownership of them.
So long as the private key of the individual gives practically unrestricted power to bind the individual to a transaction, there will be a temptation among hackers and others to gain unauthorized control of one or more individual""s private keys. Once in their possession, the unauthorized users may be able successfully to utilize the private key of the true owner for any purpose, because the private key and digital certificate associated with the corresponding public key are the presumptive indicia of personal identity.
Another example of prior art, PenOp, U.S. Pat. No. 5,544,255(1994), and continuation Ser. No. 298,991, U.S. Pat. No. 5,647,017 (1997) and related patents cited therein, adopts a completely different approach to electronic signatures. It uses a digital drawing tablet as a basis for digitally capturing the characteristics across the drawing tablet of a handwritten signature in addition to the image of the signature itself. In certain applications, the software identifies the characteristic dynamic movements of the writer""s hand across the tablet during repeated signature creation and stores them as a template which identifies the common signature characteristics of the series of such signatures. This stored information is later used for comparison purposes to identify a subsequently generated signature as authentic.
If a hash function is also used, digested, and linked to the signature, this approach, like the xe2x80x9cdigital signaturexe2x80x9d approach of the xe2x80x9casymmetric encryptionxe2x80x9d can also determine whether any changes have been made to the document since the signature was applied.
This xe2x80x9csignature dynamicsxe2x80x9d approach may avoid the massive infrastructure of the xe2x80x9cpublic key encryptionxe2x80x9d certification authorities, and the problem of conflicting legal regimes applicable to electronic signing of documents in an international or multijurisdictional setting, but nonetheless will still require the provision of a digital drawing tablet and stylus at each computer where signature is to be accomplished, as well as the related software, which can be a significant item of cost across an entire network. In addition, traditional forensic analysis applicable to handwritten signatures does not yet apply to electronic signature analysis, and it may be some time, if ever, for the legal forensic community to become adept at signature dynamics handwriting analysis. Because there is no generally accepted way at present for expert analysis of dynamic signatures, the ability to authenticate signatures from templates is arguable at best.
Furthermore, while the xe2x80x9csignature dynamicsxe2x80x9d is claimed to allow for authentication based upon a stored template of signature characteristics, the variations in signature that occur naturally each time a signature is produced make the comparison results somewhat imperfect and correspondingly error-prone as an automatic authentication scheme:
Accordingly, several objects and advantages of the invention are to provide a specific new type of electronic signature method that makes use of certificates issued by trusted parties but does not also depend upon a widespread distribution of private keys on client computers for signatures; nor upon certification authorities to keep track of valid and revoked certificates in real time; nor for a system of checking with respect to each individual transaction whether the certificate is still valid and reliable; further that uses only one signature key of a server which is shared by many users; and further, that can automatically generate and affix a date and time stamp of the server computer as proof of those parameters at the time of the signature.